“Yes Victoria, eating chocolate is unhealthy, but not when I eat it” – you might wonder just how long you can get away this kind of contradictory logic with your kids. If you’d asked Jean Piaget, one of the founding fathers of child psychology, he would probably have told you that you’ll be fine until they’re at least eight. After all, he’d observed that children younger than this age often describe things in contradictory ways, such as saying that a candle sinks because it’s round, but that a ball floats because it’s round.

Recent research has largely backed up Piaget’s view, but in a new study in Child Development, psychologists have shown that children’s recognition of logical inconsistency starts much earlier – around four years of age – when they are exposed to it in a conversational context. This makes sense, say Sabine Doebel and her colleagues, because reasoning probably evolved as a way to evaluate what we’re told by others – an especially important skill for children.

A first experiment with 74 children aged three to five involved them watching video clips of one woman asking two others a series of basic questions, like “Can you tell me about the ball you saw today?”. One woman answered all the questions in a contradictory way (“Today I saw a ball that was the biggest ball ever and it was the smallest ball ever”) whereas the other woman answered the questions in a logically consistent way (“Today I saw a ball that was the biggest ball ever and it was the softest ball ever”). Afterwards the children were asked to say which woman did not make sense.

Four-year-olds and five-year-olds, but not three-year-olds, correctly identified the women who did not make sense because they were making contradictory statements. This also affected the way the five-year-old children perceived the trustworthiness of these women. For instance, in a later part of the experiment, these children said they’d rather ask the logically consistent woman about the meaning of a new word, rather than ask the woman who’d contradicted herself.

Another experiment with more four- and five-year-olds replicated these findings in the same conversational context, but found that only the five-year-olds were unable to detect logical inconsistencies when they were attributed to books, rather than to people in conversation (to do this, the researcher presented the children with two books and, to take one example, told them that one book said someone saw a ball that was the biggest and the smallest ever, whereas the other book described someone seeing a ball that was the biggest and the softest).

Because the four-year-olds could detect logically inconsistent utterances in a conversation, but not when attributed to a book, this suggests there’s something more engaging or motivating about listening to an actual conversational exchange that improves their performance. “Put another way”, the researchers said, “the testimonial context may serve to prompt an epistemically vigilant stance, and as a result children may evaluate arguments and claims more carefully than they would otherwise”. Alternatively, perhaps they are just extra trusting of books – this would certainly chime with earlier research.

Another aspect to this second experiment was that the children also completed tests of their memory performance and executive control (they had to remember strings of numbers or recite them backwards), and those who scored higher on these tests tended to do better at detecting logical inconsistency.

A final note – although based on their average performance four-year-olds were able to identify the women who were being contradictory, not all the children at this age were able to do so, and even among five-year-olds there was plenty of room for improvement in their performance. So if you’re lucky, you might just get away a little longer with convincing your five-year-old that chocolate is bad for them but good for you, especially if you tell them that’s what a book says.